The principles of capacitive sensing (or, as called by some, electric field sensing or proximity sensing) are well known in the art. The technical paper entitled “Electric Field Sensing for Graphical Interfaces” by J. R. Smith, published in Computer Graphics I/O Devices, Issue May/June 1998, pp. 54-60 gives an overview of different techniques. The author distinguishes between distinct mechanisms he refers to as “loading mode”, “shunt mode”, and “transmit mode” which correspond to various possible electric current pathways. In the “loading mode”, an oscillating voltage signal is applied to a transmit electrode, which builds up an oscillating electric field to ground. The object to be sensed modifies the capacitance between the transmit electrode and ground. In the “shunt mode”, an oscillating voltage signal is applied to the transmit electrode, building up an electric field to a receive electrode, and the displacement current induced at the receive electrode, which is influenced by the body being sensed, is measured. In the “transmit mode”, the transmit electrode is put in contact with the user's body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling. “Shunt mode” is alternatively referred to as “coupling mode”.
The electrode whose capacitance to ground or another electrode of the system is determined is herein referred to as “sensing electrode”. It should be noted that capacitance may be determined in various ways, e.g. by measuring amplitude and phase of the current flowing in the sensing electrode, the charging time of the sensing electrode, and the like.
The to be determined capacitance of the sensing electrode is influenced by conductive elements in the vicinity of the sensing electrode. To reduce the sensitivity of the sensing electrode with respect to regions that are of no or lesser interest (such as, in case of an occupant detection system of a vehicle seat, the region underneath the seat surface), it is known to provide a so-called driven shield electrode in between the sensing electrode and the said region of no or lesser interest. During the capacitance measurement, the driven shield electrode (hereinafter referred to as “shielding electrode”) and the sensing electrode are kept at substantially the same electric potential so that the electric field between the sensing electrode and the driven shield electrode substantially cancels. Occupant detection systems featuring such a shielding electrode are described, for instance, in WO 99/28702 and EP 1 787 871. Vehicle seats may be equipped with seat heaters to increase the occupant's comfort when driving in cold weather conditions. Such seat heaters normally comprise one or more electrical heating elements, integrated in the seating portion, the side bolsters and/or the seat backrest, which dissipate heat when a current is caused to flow therein. Such heating elements represent conductive structures, which influence the electrical field of a capacitive sensing system and thus the measurements performed by that system. In conventional arrangements, to prevent the sensing electrode from coupling to the seat heating elements, the latter are arranged deeper within the seat than the electrodes of the capacitive sensing system, with the shielding electrode being interposed between the sensing electrode and the heating elements. The drawback of such an arrangement is that the heating of the seat surface becomes less effective, first, because of the increased distance to the seat surface and, second, because of the additional heat shielding effect of the electrode arrangement.